L 16 Heat and Thermodynamics [1]
World and US energy Consumption
„ What
is temperature?
„ How is it measured?
„ What is heat?
„ What is the difference between heat and
temperature?
„ Applications: engines, refrigerators, air
conditioners, human body, electric power
production systems
„ It’s all about how ENERGY is used.
The US uses about
25 % of the total
Temperature is not the whole story!
Hot
toast
Drilling
• Cake and pan just
taken out of a 400°
oven.
• Both are at 400°
• You can touch cake
but not the pan!
• You can handle toast
right out of the toaster
• Don’t burn your tongue
Engines
• After drilling into a
piece of metal, the
drill bit is very hot
• You can also get
the drill bit hot by
placing it in a torch
• Is the a difference
in the outcome?
Human engine
• Any device which uses heat to do work
• Steam engine, internal combustion engine
Burn fuel Æ boil water (steam) Æ push piston (work)
• The human body is an engine.
• Food in Æ metabolism Æ work out
• Energy in Æ
Hero’s
engine
BODY
ENGINE
Æ Energy out
• We are all subject to the laws of
thermodynamics
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Energy transfers
Internal energy & Temperature
• All systems have internal energy
• The internal energy is the sum of the
energy of all the molecules in the system
• For example- in a gas the molecules are in
random motion – each molecule has
kinetic energy (energy of motion = ½ m v2)
• If we add up all the kinetic energies of all
the molecules we get the internal energy
• All systems (living organisms and
mechanical) are continually exchanging
energy with other systems or their
environment.
System B
System A
Energy transfer examples
•
•
•
•
•
Ice melts in water
Water Æ ice in freezer
Pop cools in refrigerator
The sun warms you on an autumn day
Water is circulated through your car
engine to maintain a steady temperature
Thermodynamics:
• Is the study of heat and its transformation
into mechanical energy.
• It is a set of a few basic empirical rules
(derived from observation) that place limits
of how these transformations can occur,
and how efficiently they can be carried out.
Laws of thermodynamics
in a nutshell
Engines
energy
in
Efficiency =
Engine
Work out
Energy in
System C
Work
out
I.
You can’t get more work out than the
energy you put in (conservation of
energy).
II.
You can’t even get as much out as you put
in (engine efficiency cannot be 100%).
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Energy conversion
Temperature measurement
• We use the fact that the properties of
materials change with temperature
• For example:
Conversion and
distribution losses 30%
o Metals expand with increasing temp
o Length of liquid column expands
o Electrical resistance changes
o Pressure of a gas increases with temp
o Infrared emission from objects changes color
30 years ago almost 50% of energy was lost
as waste heat. Things are improving!
Can we trust our senses
of hot and cold?
Length of a mercury column
Mercury
column
Will both fingers feel the same temperature
when they are put in the warm water?
Temperature scales: based on
freezing and boiling points of water
Celsius
scale
100°
boiling
point
Fahrenheit
scale
212°
100°
180°
0°
freezing
point
32°
Mercury
reservoir
• The length of the Hg
column increases with
temperature
• How is the thermometer
calibrated?
• Æ temperature scales
– Fahrenheit
– Celsius
– Kelvin
Centigrade & Fahrenheit scales
• Scales are offset ( 0 °F is not 0°C)
• Celsius scale is compressed compared to
the Fahrenheit scale
• 1°C = 180/100 = 9/5 °F
• Conversion formulas:
¾TC = (5/9) × (TF – 32)
¾TF = (9/5 × TC) + 32
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Examples
Absolute zero – as cold as it gets!
1) What is the temperature in C if the
temperature is 68°F?
TC = (5/9) ×(TC – 32 ) = (5/9)×(68 – 32)
= (5/9) × (36) = 20°C
2) What is the temperature in F if the
temperature is – 10 °C?
TF = (9/5 × TC) + 32 = (9/5 × – 10) + 32
= – 18 + 32 = 14°F
Absolute zero
• There is nothing particularly significant
about 0°C or 0°F.
• Is there a temperature scale where 0 really
is ZERO? It doesn’t get any colder than this!
• YES– It is called the KELVIN scale.
• At zero Kelvin, all molecular motion stops.
• We can see this from the behavior of gases
Kelvin scale
Gas Pressure
• TK = TC + 273.15°
• One degree K = one degree C
°C
273.15 °C
As a gas is cooled, its pressure decreases. If we
imagine continuing to cool it, the P vs T plot for
all quantities of gas extrapolate to - 273.15 C
This is absolute zero!
• There are NO negative Kelvin
temperatures, zero is the minimum.
Thermal Expansion
L1
L2
Metal bar at T1
Metal bar at T2 > T1
• Expansion occurs in all dimensions
• Expansion must be taken into account when
designing roads and bridges in climates that
vary significantly from winter to summer – all
materials expand, steel, concrete, asphalt . . .
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